Hello Andy,
Thanks for the discussion. Always good to see different ways and
solutions.
Yes, that's another way to reduce the 4V problem in my case. But maybe
it is even a bit more critical. Depending on the mechanical load /
torque and temperature of the motor it seems that sometimes even 4V is
not enough to start the motor running. Once it runs, then 4V is OK. So
i need more voltage to safely start the motor. Yes yes, this
can be compensated by inserting another C in series to the feedback R
and switching another R in parallel to that R :-)
I replaced the 33 Ohm resistor by a series configuration of 5 pairs of
anti-parallel switched UF4007 diodes that were available, giving a 6V
voltage drop. It was the easiest way to optimise the circuit here. This
safely starts the motor now and allows a reasonable adjustment time now.
I'm QSYing from 136.172 to 137.7. and start to TX on 137.7, say in
DFCW-4 without resonating the antenna before. This is done
automatically now in the first DFCW element which appears a bit weaker
then at some receivers. So it takes just 4 seconds. I'm often running
the system from remote since not beeing in Heidelberg since 4 days :-)
It works :-)
Haven't done a phase measurement of the actual phase yet (with an
oscilloscope) but i estimate the reactance offset is < than +- j 1
Ohm.
73, Stefan/DK7FC
Am 24.12.2011 21:37, schrieb Andy Talbot:
In that case - you could try a non-linear amplifier, with a high gain
until the magnitude of the output voltage is around 4V then reducing.
Makie the opamp feedback R of two elements, then shunt one of these
with a pair of back to back zeners of about 4V rating. At low Vout,
the two resistors operate in series giving a high gain. As 4V output
is reached, the diodes start to conduct, removing the contribution
from the R they are shunting, and leaving the remaining series R for
reduced gain. The speed of the gain change can be modified with
another R in series with the diode arm
Bit hit and miss, perhaps, but it ought to be possible to compensate a
non-linear load by a non-linear drive network.
Andy
www.g4jnt.com
2011/12/23 Stefan Schäfer <[email protected]>:
Hi Andy,
Thanks for the hint.
I intended to use a hard drive since the motor does not rotate when supplied
with < 4 V DC. So it would always stop before the resonance is reached. That
problem may be reduced by increasing the gain of the input stage but then i
expect oscillation problems and small currents running through the motor all
the time...
73, Stefan
Am 23.12.2011 20:24, schrieb Andy Talbot:
Stefan..
An idea. You have implemented a bang-bang driver with a dead zone.
Switched hard on in either direction, or stopped close to the middle.
Change the circuit so you linearly amplify the output from the phase
detector, and make youself a bidirectional bridge driver, using say an
op-amp and NPN-PNP pair - as in audio amplifiers. Or it may even be
possible to use an audio amplifier chip if the gain can be brought
down low enough..
Now, the motor will automatically slow as the phase error is reduced,
and you 'shouldn't' have to include any dead zone.
You may have to compensate the loop to prevent overshoot or high
frequency instabilities if there is too much mechanical lag in the
system or inertia, but that is something that can be done pure with CR
networks.
Andy
www.g4jnt.com
2011/12/23 Stefan Schäfer<[email protected]>:
Jim,
Another thought:
Am 23.12.2011 12:17, schrieb James Moritz:
[...] I guess the inertia of the motor and gearbox will produce some
hysteresis, moving the variometer to a very slightly over-adjusted
position
after the motor drive is removed, which will also help to prevent the
system
continuously searching around the correct tuning point.[...]
This effect can be compensated by choosing the dead band treshold value.
Then the motor gets stopped a bit before the resonance point and will
exactly land on it. As good as having an infinitesimal small dead band
;-)
Anyway we are talking about phase angles which are very very small, much
below that what could become critical for a PA.
73, Stefan/DK7FC
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